Conventionally, mineral acids (sulfuric acid, phosphoric acid, etc.) and sulfonic acids (p-toluene sulfonic acid, methane sulfonic acid, trifluoromethane sulfonic acid, etc.) usually are used as acid catalysts used in the reaction between alcohol and α,β-unsaturated carboxylic acids or lower esters thereof for production of α,β-unsaturated carboxylic acid esters.
However, in the case where such a Brønsted acid is used as a catalyst, complicated steps for removing a catalyst, for example, neutralization, washing with water, or adsorption, are needed after the reaction, and waste products are generated in a large amount. Besides, catalyst residues remain in the obtained carboxylic acid ester, and sometimes cause a problem of corrosion of a metal that the catalyst residues are in contact with, depending on the manner in which the ester obtained is used.
As a solution for such a problem, a method for producing an α,β-unsaturated carboxylic acid ester has been proposed in which one of various “solid acid” catalysts is used. The use of the “solid acid” as a heterogeneous catalyst facilitates the separation of the catalyst from a reaction product, thereby reducing waste products generated by neutralization, washing with water, etc.
As such a solid acid catalyst, the following have been proposed: ion exchange resins (sulfonated product of styrene-divinylbenzene copolymer, etc.) (see Patent Documents 1 and 2 shown below); solid super strong acids such as phosphotungstic acid (see Patent Document 3 shown below); and fluororesins containing sulfonic acid (“Nafion” produced by Du-Pont, etc.).
However, none of these solid acid catalysts exhibits satisfactory catalytic activity as a catalyst for α,β-unsaturated carboxylic acid esters, and tend to cause generation of by-products. Examples of the by-product include elimination reaction products (olefins generated when water is eliminated from monomolecular alcohols), etherified products (ethers generated by dehydrocondensation of bimolecular alcohols), and addition products (adducts generated by addition of alcohols to α,β-unsaturated groups). Therefore, these solid acid catalysts have a problem that the obtained α,β-unsaturated carboxylic acid esters have low purity.
Besides, there also is a problem that acidic components such as sulfur oxides that have eluted upon decomposition of a catalyst tend to remain, even though the amount is smaller than that in the case of a Brønsted acid.    Patent Document 1: U.S. Pat. No. 6,087,527    Patent Document 2: WO 90/08127    Patent Document 3: U.S. Pat. No. 5,827,939